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Author: search.filters.author.Subramanian, Kandaswamy

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    Zeldovich approximation and the probability distribution for the smoothed density field in the nonlinear regime
    (2014-11-23) Padmanabhan, T.; Subramanian, Kandaswamy
    The study of large-scale structure in the Universe is often based on the observed density distribution of matter smoothed by a suitable filter function. The probability distribution for this smoothed density field in the nonlinear regime is studied using the Zeldovich approximation. When the shear term of the velocity field is not too large, one can obtain a reasonably good analytic approximation to this probability distribution. The properties of this distribution are discussed and compared with other attempts along similar lines.
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    Cosmic Microwave Background Trispectrum and Primordial Magnetic Field Limits
    (2012-06-08) Trivedi, Pranjal; Seshadri, T. R.; Subramanian, Kandaswamy
    Primordial magnetic fields will generate non-Gaussian signals in the cosmic microwave background (CMB) as magnetic stresses and the temperature anisotropy they induce depend quadratically on the magnetic field. We compute a new measure of magnetic non-Gaussianity, the CMB trispectrum, on large angular scales, sourced via the Sachs-Wolfe effect. The trispectra induced by magnetic energy density and by magnetic scalar anisotropic stress are found to have typical magnitudes of approximately a few times 10 29 and 10 19, respectively. Observational limits on CMB non-Gaussianity from WMAP data allow us to conservatively set upper limits of a nG, and plausibly sub-nG, on the present value of the primordial cosmic magnetic field. This represents the tightest limit so far on the strength of primordial magnetic fields, on Mpc scales, and is better than limits from the CMB bispectrum and all modes in the CMB power spectrum. Thus, the CMB trispectrum is a new and more sensitive probe of primordial magnetic fields on large scales.
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    Magnetic helicity in stellar dynamos : new numerical experiments
    (2001-01-04) Axel, Brandenburg; Wolfgang, Dobler; Subramanian, Kandaswamy
    The theory of large scale dynamos is reviewed with particular emphasis on the magnetic helicity constraint in the presence of closed and open boundaries. In the presence of closed or periodic boundaries, helical dynamos respond to the helicity constraint by developing small scale separation in the kinematic regime, and by showing long time scales in the nonlinear regime where the scale separation has grown to the maximum possible value. A resistively limited evolution towards saturation is also found at intermediate scales before the largest scale of the system is reached. Larger aspect ratios can give rise to different structures of the mean field which are obtained at early times, but the final saturation field strength is still decreasing with decreasing resistivity. In the presence of shear, cyclic magnetic fields are found whose period is increasing with decreasing resistivity, but the saturation energy of the mean field is in strong super-equipartition with the turbulent energy. It is shown that artificially induced losses of small scale field of opposite sign of magnetic helicity as the large scale field can, at least in principle, accelerate the production of large scale (poloidal) field. Based on mean field models with an outer potential field boundary condition in spherical geometry, we verify that the sign of the magnetic helicity flux from the large scale field agrees with the sign of α. For solar parameters, typical magnetic helicity fluxes lie around 1047 Mx2 per cycle.
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    Magnetic helicity in galactic dynamos
    (2002-07-06) Subramanian, Kandaswamy
    Magnetic fields correlated on kiloparsec scales are seen in spiral galaxies. Their origin could be due to amplification of a small seed field by a turbulent galactic dynamo. We review the current status of the galactic dynamo, especially the constraints imposed by magnetic helicity conservation. We estimate the minimal strength of the large-scale magnetic field which could arise inspite of the helicity constraint
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    Small-scale microwave background anisotropies arising from tangled promordial magnetic fields
    (2002-03-12) Subramanian, Kandaswamy; Barrow, John D.
    An inhomogeneous cosmologicalmagnetic field creates vortical perturbations that survive Silk damping onmuch smaller scales than compressionalmodes. This ensures that there is no sharp cut-off in anisotropy on arcminute scales. As we had pointed out earlier, tangled magnetic fields, if they exist, will then be a potentially important contributor to small-angular-scale cosmic microwave background radiation anisotropies. Several ongoing and new experiments are expected to probe the very small angular scales, corresponding tomultipoles with l >1000. In view of this observational focus, we revisit the predicted signals arising from primordial tangled magnetic fields, for different spectra and different cosmological parameters. We also identify a new regime, where the photon mean-free path exceeds the scale of the perturbation, which dominates the predicted signal at very high l. A scale-invariant spectrum of tangled fields which redshifts to a present value B0 = 3 × 10−9 G produces temperature anisotropies at the 10-µK level between l ∼1000 and 3000. Larger signals result if the universe is lambda- dominated, if the baryon density is larger, or if the spectral index ofmagnetic tangles is steeper, n >−3. The signalwill also have non-Gaussian statistics.We predict the distinctive formof the increased power expected in the microwave background at high l in the presence of significant tangled magnetic fields. We may be on the verge of detecting or ruling out the presence of tangledmagnetic fields that are strong enough to influence the formation of large-scale structure in the Universe.
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    Hyperdiffusion in non-linear, large and small-scale turbulent dynamos
    (2011-07-05) Subramanian, Kandaswamy
    The generation of large-scale magnetic fields is generically accompanied by the more rapid growth of small-scale fields. The growing Lorentz force due to these fields back reacts on the turbulence to saturate the mean-field and small-scale dynamos. For the mean-field dynamo, in a quasi-linear treatment of this saturation, it is generally thought that, while the alpha-effect gets renormalised and suppressed by non-linear effects, the turbulent diffusion is left unchanged. We show here that this is not true and the effect of the Lorentz forces, is also to generate additional non-linear hyperdiffusion of the mean field. A combination of such non-linear hyperdiffusion with diffusion at small scales, also arises in a similar treatment of small-scale dynamos, and is crucial to understand its saturation.
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    Effects of curvature and interactions on the dynamics of the deconfinement phase transition
    (2011-07-06) Subramanian, Kandaswamy; Seshadri, T. R.; Barrow, John D.
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    Magnetic quenching of alpha and diffusity tensors in helical turbulence
    (2011-07-06) Brandenburg, Axel; Biman, B.; Subramanian, Kandaswamy; et al.
    We study the implications of primordial magnetic fields for the thermal and ionization history of the post-recombination era. In particular we compute the effects of dissipation of primordial magnetic fields owing to ambipolar diffusion and decaying turbulence in the intergalactic medium (IGM) and the collapsing halos and compute the effects of the altered thermal and ionization history on the formation of molecular hydrogen.We show that, for magnetic field strengths in the range 2×10−10 G < ∼ B0 < ∼ 2× 10−9 G, the molecular hydrogen fraction in IGM and collapsing halo can increase by a factor 5 to 1000 over the case with no magnetic fields. We discuss the implication of the increased molecular hydrogen fraction on the radiative transfer of UV photons and the formation of first structures in the universe
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    Nonlinear current helicity fluxes in turbulent dynamos and alpha quenching
    (2011-07-06) Subramanian, Kandaswamy; Brandenburg, Axel
    Large scale dynamos produce small scale current helicity as a waste product that quenches the large scale dynamo process (alpha effect). This quenching can be catastrophic (i.e. intensify with magnetic Reynolds number) unless one has fluxes of small scale magnetic (or current) helicity out of the system. We derive the form of helicity fluxes in turbulent dynamos, taking also into account the nonlinear effects of Lorentz forces due to fluctuating fields. We confirm the form of an earlier derived magnetic helicity flux term, and also show that it is not renormalized by the small scale magnetic field, just like turbulent diffusion. Additional nonlinear fluxes are identified, which are driven by the anisotropic and antisymmetric parts of the magnetic correlations. These could provide further ways for turbulent dynamos to transport out small scale magnetic helicity, so as to avoid catastrophic quenching.